Selective naphtha desulfurization process and catalyst

ABSTRACT

A process and catalyst for the selective hydrodesulfurization of a naphtha containing olefins. The process produces a naphtha stream having a reduced concentration of sulfur while maintaining the maximum concentration of olefins.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a Division of copending application Ser. No.10/875,117 filed Jun. 23, 2004, the contents of which are herebyincorporated by reference in its entirety.

FIELD OF THE INVENTION

The field of art to which this invention pertains is the selectivehydrodesulfurization of a naphtha containing olefins. The desiredproduct is a naphtha stream having a reduced concentration of sulfurwhile maintaining the maximum concentration of olefins.

BACKGROUND OF THE INVENTION

Naphtha streams are one of the primary products in the refining of crudeoil. These streams are blended to provide a gasoline pool which ismarketed as motor fuel. Naphtha streams particularly those streams whichare products of a thermal or catalytic cracking process such as cokingor fluidized catalytic cracking contain undesirable high levels ofsulfur and desirable olefin compounds. The valuable olefins contributeto the desirable characteristic of a high octane fuel in the resultinggasoline pool and thus it is desirable not to saturate the high octaneolefins to lower octane paraffins during hydrodesulfurization. There isa continuing need for catalysts having improved properties for thedesulfurization of naphtha streams in order that the sulfurconcentration of the cracked naphtha can be reduced. The prior art hastaught hydrodesulfurization catalysts and processes for desulfurizingnaphtha feed streams while striving to minimize the saturation of theolefin compounds. While there are commercially successfulhydrodesulfurization catalysts in use today, there is a continuing needfor improved catalysts that are capable of combining a high level ofdesulfurization with a minimum of olefin saturation.

U.S. Pat. No. 6,126,814 (Lapinski et al.) discloses a process forhydrodesulfurizing a naphtha feedstream using a catalyst comprisingmolybdenum and cobalt and having an average median pore diameter fromabout 60 Å to 200 Å, a cobalt to molybdenum atomic ratio of about 0.1 toabout 1, a molybdenum oxide surface concentration of about 0.5×10⁻⁴ toabout 3×10⁻⁴ g molybdenum oxide/m² and an average particle size of lessthan 2 mm in diameter.

U.S. Pat. No. 6,177,381 (Jensen et al.) discloses a layered catalystcomposition comprising an inner core such as alpha alumina and an outerlayer bonded to the inner core composed of an outer refractory inorganicoxide such as gamma alumina. The outer layer has uniformly dispersedthereon a platinum group metal such as platinum and a promoter metalsuch as tin. The composition also contains a modifier metal such aslithium. The catalyst shows improved durability and selectivity fordehydrogenating hydrocarbons. The patent also discloses that thiscatalyst is useful for the hydrogenation of hydrocarbons.

U.S. Pat. No. 6,673,237 (Liu et al.) discloses a process for theselective desulfurization of naphtha feed streams utilizing a monolithichoneycomb catalyst bed.

U.S. Pat. No. 4,716,143 (Imai et al.) discloses a surface impregnatedcatalytic composite comprising a platinum group metal component.

BRIEF SUMMARY OF THE INVENTION

The present invention is a selective naphtha desulfurization processutilizing a hydrodesulfurization catalyst having the desulfurizationmetal dispersed in a thin outer layer of the catalyst. In one embodimentof the present invention, the hydrodesulfurization catalyst is a layeredcomposition comprising an inner core and an outer layer comprising aninorganic oxide bonded to the inner core wherein the outer layercontains a desulfurization metal dispersed in the outer layer. In ananother embodiment of the present invention, the hydrodesulfurizationcatalyst is surface impregnated with a desulfurization metal such thatthe average concentration of the surface impregnated desulfurizationmetal on the surface layer having a thickness of from about 40 to about400 microns is at least two times the concentration of the respectivedesulfurization metal in the center core of the support.

BRIEF DESCRIPTION OF THE DRAWING

The FIGURE shows a plot of the hydrodesulfurization selectivity of thepresent invention compared with the prior art.

DETAILED DESCRIPTION OF THE INVENTION

Naphtha feedstocks suitable for use in the present invention cancomprise any one or more refinery stream boiling in the range from about38° C. (100° F.) to about 232° C. (450° F.) at atmospheric pressure. Thenaphtha feedstock generally contains cracked naphtha which usuallycomprises fluid catalytic cracking unit naphtha (FCC naphtha), cokernaphtha, hydrocracker naphtha and gasoline blending components fromother sources wherein a naphtha boiling range stream can be produced.FCC naphtha and coker naphtha are generally more olefinic naphthas sincethey are products of catalytic and thermal cracking reactions, and aremore preferred feedstocks for use in the present invention.

The naphtha feedstock, preferably a cracked naphtha, generally containsnot only paraffin, naphthenes, and aromatics, but also unsaturates, suchas open-chain and cyclic olefins, dienes and cyclic hydrocarbons witholefinic side chains. The cracked naphtha feedstocks generally containan overall olefins concentration ranging as high as about 60 weightpercent. The cracked naphtha feedstock can comprise a dieneconcentration of as much as 15 weight percent. High diene concentrationscan result in a gasoline product with poor stability and color. Thecracked naphtha feedstock sulfur content will generally range from about0.05 to about 0.7 weight percent based on the weight of the feedstock.Nitrogen content will generally range from about 5 wppm to about 500wppm.

There are many hydrodesulfurization catalysts in the prior art but alongwith their ability to desulfurize naphtha boiling range hydrocarbonsthey successfully hydrogenate the olefins which may be present. Forenvironmental reasons, the naphtha must be desulfurized but the olefinscontribute to a high octane rating and therefore it is highly desirableto retain the highest olefin concentration possible in the desulfurizednaphtha. Many of the approaches to naphtha desulfurization have focusedon modifying traditional hydrotreating processes using less severeoperating conditions and catalysts that selectively remove sulfur butleave the bulk of the olefins unreacted.

It has unexpectedly been discovered that hydrotreating catalysts inwhich the metal loading is restricted to the outer layer of thecatalysts are more selective for hydrodesulfurization compared to olefinsaturation than catalysts in which the metal is uniformly distributed.In accordance with the present invention, the catalysts preferablycontain desulfurization metals selected from the group consisting ofcobalt, nickel, molybdenum and tungsten.

The catalyst support material utilized in one embodiment of the presentinvention is a layered composition comprising an inner core composed ofa material which has substantially lower adsorptive capacity forcatalytic metal precursors, relative to the outer layer. Some of theinner core materials are also not substantially penetrated by liquidhydrocarbons. Examples of the inner core material include, but are notlimited to, refractory inorganic oxides, silicon carbide and metals.Examples of refractory inorganic oxides include without limitation alphaalumina, theta alumina, cordierite, zirconia, titania and mixturesthereof. A preferred inorganic oxide is alpha alumina.

The materials which form the inner core can be formed into a variety ofshapes such as pellets, extrudates, spheres or irregularly shapedparticles although not all materials can be formed into each shape.Preparation of the inner core can be done by means known in the art suchas oil dropping, pressure molding, metal forming, pelletizing,granulation, extrusion, rolling methods and marumerizing. A sphericalinner core is preferred. The inner core whether spherical or notpreferably has an effective diameter of about 0.05 mm to about 5 mm andmore preferably from about 0.8 mm to about 3 mm. For a non-sphericalinner core, effective diameter is defined as the diameter the shapedarticle would have if it were molded into a sphere. Once the inner coreis prepared, it is calcined at a temperature of about 400° C. to about1500° C.

The inner core is next coated with a layer of a refractory inorganicoxide which is different from the inorganic oxide which may be used asthe inner core and will be referred to as the outer refractory inorganicoxide. This outer refractory oxide is one which has good porosity, has asurface area of at least 20 m²/g, and preferably at least 50 m²/g, anapparent bulk density of about 0.2 g/ml to about 1.5 g/ml and is chosenfrom the group consisting of gamma alumina, delta alumina, eta alumina,theta alumina, silica/alumina, zeolites, non-zeolitic molecular sieves(NZMS), titania, zirconia and mixtures thereof. It should be pointed outthat silica/alumina is not a physical mixture of silica and alumina butmeans an acidic and an amorphous material that has been cogelled orcoprecipitated. This term is well known in the art, see e.g., U.S. pat.No. 3,909,450; U.S. Pat. No. 3,274,124 and U.S. Pat. No. 4,988,659.Examples of zeolites include, but are not limited to, zeolite Y, zeoliteX, zeolite L, zeolite beta, ferrierite, MFI, mordenite and erionite.Non-zeolitic molecular sieves (NZMS) are those molecular sieves whichcontain elements other than aluminum and silicon and includesilicoaluminophosphates (SAPOs) described in U.S. Pat. No. 4,440,871,ELAPSOs described in U.S. Pat. No. 4,793,984, MeAPOs described in U.S.Pat. No. 4,567,029 all of which are incorporated by reference. Preferredrefractory inorganic oxides for the outer layer are gamma and etaalumina.

A way of preparing a gamma alumina is by the well-known oil drop methodwhich is described in U.S. Pat. No. 2,620,314 which is incorporated byreference. The oil drop method comprises forming an aluminum hydrosol byany of the techniques taught in the art and preferably by reactingaluminum metal with hydrochloric acid; combining the hydrosol with asuitable gelling agent, e.g., hexamethylenetetraamine, and dropping theresultant mixture into an oil bath maintained at elevated temperatures(about 93° C.). The droplets of the mixture remain in the oil bath untilthey set and form hydrogel spheres. The spheres are then continuouslywithdrawn from the oil bath and typically subjected to specific agingand drying treatments in oil and ammoniacal solutions to further improvetheir physical characteristics. The resulting aged and gelled spheresare then washed and dried at a relatively low temperature of about 80°C. to 260° C. and then calcined at a temperature of about 455° C. to705° C. for a period of about 1 to about 20 hours. This treatmenteffects conversion of the hydrogel to the corresponding crystallinegamma alumina.

The outer layer is applied by forming a slurry of the outer refractoryoxide and then coating the inner core with the slurry by means wellknown in the art. Slurries of inorganic oxides can be prepared by meanswell known in the art which usually involve the use of a peptizingagent. For example, any of the transitional aluminas can be mixed withwater and an acid such as nitric, hydrochloric, or sulfuric to give aslurry. Alternatively, an aluminum sol can be made by for example,dissolving aluminum metal in hydrochloric acid and then mixing thealuminum sol with the alumina powder.

It is preferred that the slurry contain an organic bonding agent whichaids in the adhesion of the layer material to the inner core. Examplesof this organic bonding agent include but are not limited to polyvinylalcohol (PVA), hydroxyl propyl cellulose, methyl cellulose and carboxymethyl cellulose. The amount of organic bonding agent which is added tothe slurry will vary considerably from about 0.1 weight percent to about3 weight percent of the slurry. How strongly the outer layer is bondedto the inner core can be measured by the amount of layer material lostduring an attrition test, i.e., attrition loss. Loss of the secondrefractory oxide by attrition is measured by agitating the catalyst,collecting the fines and calculating an attrition loss. It has beenfound that by using an organic bonding agent as described above, theattrition loss is less than about 10 weight percent of the outer layer.Finally, the thickness of the outer layer varies from about 40 to about400 microns, preferably from about 40 microns to about 300 microns andmore preferably from about 45 microns to about 200 microns.

Depending on the particle size of the outer refractory inorganic oxide,it may be necessary to mill the slurry in order to reduce the particlesize and simultaneously give a narrower particle size distribution. Thiscan be done by means known in the art such as ball milling for times ofabout 30 minutes to about 5 hours and preferably from about 1.5 to about3 hours. It has been found that using a slurry with a narrow particlesize distribution improves the bonding of the outer layer to the innercore.

The slurry may also contain an inorganic bonding agent selected from analumina bonding agent, a silica bonding agent or mixtures thereof.Examples of silica bonding agents include silica sol and silica gel,while examples of alumina bonding agents include alumina sol, boehmiteand aluminum nitrate. The inorganic bonding agents are converted toalumina or silica in the finished composition. The amount of inorganicbonding agent varies from about 2 to about 15 weight percent as theoxide, and based on the weight of the slurry.

Coating of the inner core with the slurry can be accomplished by meanssuch as rolling, dipping, spraying, etc. One preferred techniqueinvolves using a fixed fluidized bed of inner core particles andspraying the slurry into the bed to coat the particles evenly. Thethickness of the layer can vary considerably, but usually is from about40 to about 400 microns preferably from about 40 to about 300 micronsand most preferably from about 50 microns to about 200 microns. Once theinner core is coated with the layer of outer refractory inorganic oxide,the resultant layered support is dried at a temperature of about 100° C.to about 320° C. for a time of about 1 to about 24 hours and thencalcined at a temperature of about 400° C. to about 900° C. for a timeof about 0.5 to about 10 hours to effectively bond the outer layer tothe inner core and provide a layered catalyst support. Of course, thedrying and calcining steps can be combined into one step.

When the inner core is composed of a refractory inorganic oxide (innerrefractory oxide), it is necessary that the outer refractory inorganicoxide be different from the inner refractory oxide. Additionally, it isrequired that the inner refractory inorganic oxide have a substantiallylower adsorptive capacity for catalytic metal precursors relative to theouter refractory inorganic oxide.

Having obtained the layered catalyst support, catalytic metals can bedispersed on the layered support by means known in the art. Thesecatalytic metal components can be deposited on the layered support inany suitable manner known in the art. One method involves impregnatingthe layered support with a solution (preferably aqueous) of adecomposable compound of the metal or metals. By decomposable is meantthat upon heating the metal compound is converted to the metal or metaloxide with the release of byproducts. The metals of the catalyst of thepresent invention can be deposited or incorporated upon the support byany suitable conventional means, such as by impregnation employingheat-decomposable salts of the desired hydrogenation metals or othermethods known to those skilled in the art such as ion-exchange, withimpregnation methods being preferred.

Impregnation of the hydrogenation metals on the catalyst support can beperformed using incipient wetness techniques. The catalyst support isprecalcined and the amount of water to be added to just wet all of thesupport is determined. The aqueous impregnation solutions are added suchthat the aqueous solution contains the total amount of hydrogenationcomponent metal or metals to be deposited on the given mass of support.Impregnation can be performed for each metal separately, including anintervening drying step between impregnation, or as a singleco-impregnation step. The saturated support can then be separated,drained and dried in preparation for calcination which is generallyperformed at a temperature from about 260° C. (500° F.) to about 648° C.(1200° F.), or more preferably from about 426° C. (800° F.) to about593° C. (1100° F.). The outer refractory inorganic oxide may beimpregnated or otherwise associated with desulfurization metals beforebeing deposited on the inner refractory oxide or core. In any event, thedesulfurization metals are preferably present on the outer refractoryinorganic oxide in an amount from about 2 to about 20 weight percent.

In accordance with another embodiment of the present invention, thecatalytic composite comprises a catalytic desulfurization metal selectedfrom the group consisting of cobalt, nickel, molybdenum and tungsten ona refractory inorganic oxide support wherein the desulfurization metalis surface impregnated such that the average concentration of thesurface impregnated desulfurization metal on the surface layer having athickness from about 40 to about 400 microns is at least about two timesthe concentration of the respective desulfurization metal in the centercore of the support.

An essential feature of this embodiment of the present invention is thatthe desulfurization metal or metals are surface impregnated upon acatalytic support material and that substantially all of thedesulfurization metal is located within at most a 400 micron exteriorlayer of the catalyst support. It is to be understood that the term“exterior” is defined as the outermost layer of the catalyst particle.By “layer” it is meant a stratum of substantially uniform thickness.

A desulfurization metal is considered to be surface impregnated when theaverage concentration of the metal or metals within a 40 to 400 micronexterior layer of the catalyst is at least about two times the averageconcentration of the same metal component in the center core of thecatalyst. By “substantially all” it is meant that at least about 75% ofthe surface impregnated metal component(s) in question. The surfaceimpregnated metal concentration then tapers off as the center of thesupport is approached. The actual gradient of the desulfurizationmetal(s) within the catalyst support varies depending upon the exactmanufacturing method employed to fabricate the catalyst. Therefore,distribution of the desulfurization metal is best defined as being bothsurface impregnated and substantially all located within at most the 400micron exterior layer of the support.

The support material preferably has a nominal diameter of 850 microns ormore. For a catalyst support material having a diameter of 850 microns,the exterior layer wherein 75% of the surface impregnated components arelocated will approach 100 microns. The exterior layer wherein 75% of thesurface-impregnated metal(s) are located will approach a maximum valueof 400 microns as the diameter of the catalyst support increases beyond2000 microns.

Although it is not understood completely and not wishing to be bound byany particular theory, it is believed that by restricting substantiallyall of the surface impregnated metal(s) to at most a 400 micron exteriorlayer of the catalyst support, more facile and selective access to thesecatalytic sites is achieved, allowing the hydrocarbon reactants andproducts shorter diffusion paths. By decreasing the length of thediffusion paths the reactants and products have a shorter and optimumresidence time in the catalyst particle thereby successfully achievingthe desulfurization reaction while simultaneously minimizing thesaturation or hydrogenation of olefin components of the fresh feed. Thisresults in an increase in selectivity to desired product of adesulfurized naphtha while maximizing the retention of the olefins.

The catalyst support for this embodiment of the present invention may beselected from the inorganic oxides disclosed and taught hereinabove assuitable for the outer refractory inorganic oxide in another embodimentof the present invention. Preferred refractory inorganic oxide supportmaterials for the instant embodiment are gamma and eta alumina. In anembodiment desulfurization metal(s) may be incorporated into thecatalytic composite of the invention by any means suitable to result insurface impregnation of the metal(s) wherein substantially all of thesurface impregnated metal(s) is located within at most a 400 micron wideexterior layer of the catalyst support particle. The surfaceimpregnation may be conducted by utilizing any known technique whichachieves the necessary distribution of metals as described herein. Onemethod for the surface impregnation of metals on a desulfurizationcatalyst is to adjust the pH of the impregnation solution to control thelocation of the metal components. Another method for the surfaceimpregnation is to restrict the total volume of the impregnationsolution in order to restrict the penetration of solution and therebymetals into the support particle. After the desulfurization metalcomponents have been surface impregnated on the catalyst support, theresulting catalyst composite will generally be dried at a temperaturefrom about 100° C. to about 150° C. and then calcined at a temperaturefrom about 300° C. to about 650° C. The finished surface impregnatedcatalyst preferably contains desulfurization metals in an amount fromabout 2 to about 20 weight percent.

Hydrodesulfurization conditions preferably include a temperature fromabout 240° C. (400° F.) to about 399° C. (750° F.) and a pressure fromabout 790 kPa (100 psig) to about 4 MPa (500 psig). Thehydrodesulfurization process using the catalysts of the presentinvention typically begins with a cracked naphtha feedstock preheatingstep. The charge stock is preferably preheated in a feed/effluent heatexchanger prior to entering a fired furnace for final preheating to atargeted reaction zone inlet temperature. The feedstock can be contactedwith a hydrogen-rich gaseous stream prior to, during or afterpreheating. The hydrogen-rich stream may also be added in thehydrodesulfurization reaction zone. The hydrogen stream can be purehydrogen or can be in admixture with other components found in refineryhydrogen streams. It is preferred that the hydrogen stream have little,if any, hydrogen sulfide. The hydrogen stream purity is preferably atleast about 65 volume percent hydrogen and more preferably at least 75volume percent hydrogen for best results.

The hydrodesulfurization reaction zone can consist of one or more fixedbed reactors each of which can comprise a plurality of catalyst beds.Since some olefin saturation will take place and the olefin saturationand the desulfurization reaction are generally exothermic, consequentlyinterstage cooling between fixed bed reactors or between catalyst bedsin the same reactor shell can be employed. A portion of the heatgenerated from the hydrodesulfurization process can be recovered andwhere this option is not available, cooling may be achieved withheat-exchange with the hydrogen quench stream, air or cooling water.

EXAMPLE

A catalyst was prepared by extruding a comulled dough containing cobalt,molybdenum and alumina to form 3.17 mm (⅛″) tri-lobe extrudate particlescontaining 1 weight percent cobalt and 3.4 weight percent molybdenum.The metals were uniformly dispersed throughout each catalyst particle.This resulting catalyst is identified as Catalyst A and is not acatalyst of the present invention.

A portion of Catalyst A was crushed to produce catalyst particlesranging in nominal diameter from 1.41 mm (0.0937 inches) to 2.38 mm(0.937 inches) which catalyst is identified as Catalyst B and also isnot a catalyst of the present invention.

A batch of spherical support material containing a low surface area coreof cordierite with a surface layer coating of alumina with a thicknessof 100 microns (0.1 mm) was prepared and had a nominal diameter of 2000microns (0.08 inches). This resulting spherical support material wasimpregnated to produce a catalyst having an alumina metals loading of 1weight percent cobalt and 3.4 weight percent molybdenum. This resultingcatalyst is identified as Catalyst C and is a catalyst of one embodimentof the present invention.

An olefin containing naphtha feedstock was selected to test thehereinabove described catalysts and contained a 50/50 volumetric blendof intermediate cracked naphtha and heavy cracked naphtha which blendcontained about 2200 wppm sulfur and about 24 weight percent olefins.

Each of the test catalysts was presulfided in an identical manner andtested in a hydrodesulfurization reaction zone with the above describednaphtha feedstock at conditions including a pressure of 1800 kPa (250psig), a liquid hourly space velocity (LHSV) of about 3 and atemperature of about 274° C. (525° F.). After a line out period,Catalyst A produced a product naphtha containing a sulfur concentrationof about 250 wppm but the olefin concentration was reduced from 24weight percent olefins to 18.5 weight percent olefins. Catalyst Bproduced a product naphtha containing a sulfur concentration of about250 wppm while the olefin concentration was reduced from 24 weightpercent to 19.5 weight percent. At the initial test conditions includingan inlet temperature of about 274° C. (525° F.), Catalyst C produced aproduct naphtha containing about 600 wppm sulfur but having essentiallyno reduction in olefin concentration. During the test for Catalyst C,the reactor inlet temperature was then increased from about 274° C.(525° F.) to about 296° C. (565° F.) and the product sulfurconcentration was reduced to about 250 wppm while the olefinconcentration was only reduced from 24 weight percent to 20.1 weightpercent.

Although the inlet temperature for Catalyst C was higher than forCatalyst A and Catalyst B to achieve similar product sulfurconcentrations, the highly sought characteristic of high olefinretention was observed. In order to demonstrate the olefin retentioncharacteristics, a series of selectivities were calculated for eachcatalyst. The selectivity was defined as the sulfur conversion dividedby the olefin conversion and multiplied by 100 for convenience. Thesulfur conversion is further defined as the feed sulfur minus productsulfur divided by the feed sulfur. The olefin conversion is also furtherdefined as feed olefin minus product olefin divided by the feed olefin.The resulting calculated selectivities for the three tested catalystsare plotted in the FIGURE as selectivity versus time on stream in hours.From the FIGURE, it is apparent that with a constant desulfurizationlevel, the olefin retention of Catalyst A is the lowest of the threecatalyst tested. Catalyst B will be noted to have a higher olefinretention in the product than Catalyst A. The FIGURE also shows thatCatalyst C, the catalyst of the present invention, possesses the highestolefin retention in the product of the three catalysts tested.Therefore, the present invention successfully achieves thedesulfurization of naphtha containing olefins while preserving a greaterconcentration of olefins in the desulfurized product naphtha.

The foregoing description, example and FIGURE clearly illustrate theadvantages encompassed by the present invention and the benefits to beafforded with the use thereof.

1. A process for hydrodesulfurizing a naphtha feedstream containingolefins without excessive olefin saturation which process comprises: a)reacting an olefin containing naphtha feedstream in ahydrodesulfurization zone containing a hydrodesulfurization catalystwhich catalyst is a layered catalyst composition comprising an innercore and an outer layer comprising an inorganic oxide bonded to theinner core wherein the outer layer has at least one desulfurizationmetal selected from the group consisting of cobalt, nickel, molybdenumand tungsten uniformly dispersed thereon; and b) recovering a naphthastream having a reduced sulfur concentration.
 2. The process of claim 1wherein the inner core is selected from the group consisting of alphaalumina, theta alumina, silicon carbide, metals, cordierite, zirconia,titania and mixtures thereof.
 3. The process of claim 1 wherein theouter layer comprising an inorganic oxide is selected from the groupconsisting of gamma alumina, delta alumina, theta alumina,silica-alumina, zeolites, non-zeolitic molecular sieves, titania andmixtures thereof.
 4. The process of claim 1 wherein the outer layer hasa thickness from about 40 to about 400 micrometers.
 5. The process ofclaim 1 wherein the hydrodesulfurization zone is operated at conditionsincluding a temperature from about 204° C. (400° F.) to about 399° C.(750° F.) and a pressure from about 790 kPa (100 psig) to about 3950 kPa(500 psig).
 6. The process of claim 1 wherein the naphtha feedstreamboils in the range from about 38° C. (100° F.) to about 232° C. (450°F.).
 7. The process of claim 1 wherein the inner core has a loweradsorptive capacity for catalytic metal precursors relative to the outerlayer comprising an inorganic oxide.
 8. A process for hydrodesulfurizingnaphtha feedstream containing olefins without excessive olefinsaturation which process comprises: a) reacting an olefin containingnaphtha feedstream boiling in the range from about 38° C. (100° F.) toabout 232° C. (450° F.) in a hydrodesulfurization zone containing ahydrodesulfurization catalyst having a layered catalyst compositioncomprising an inner core and an outer layer comprising an inorganicoxide bonded to the inner core wherein the outer layer has a thicknessfrom about 40 to about 400 micrometers and comprises cobalt andmolybdenum uniformly dispersed thereon; and b) recovering a naphthastream having a reduced sulfur concentration.
 9. The process of claim 8wherein the inner core has a lower adsorptive capacity for catalyticmetal precursors relative to the outer layer comprising an inorganicoxide.
 10. The process of claim 8 wherein the inner core is selectedfrom the group consisting of alpha alumina, theta alumina, siliconcarbide, metals, cordierite, zirconia, titania and mixtures thereof. 11.The process of claim 8 wherein the outer layer comprising an inorganicoxide is selected from the group consisting of gamma alumina, deltaalumina, theta alumina, silica-alumina, zeolites, non-zeolitic molecularsieves, titania and mixtures thereof.
 12. The process of claim 8 whereinthe hydrodesulfurization zone is operated at conditions including atemperature from about 204° C. (400° F.) to about 399° C. (750° F.) anda pressure from about 790 kPa (100 psig) to about 3950 kPa (500 psig).13. A process for desulfurizing a naphtha feedstream containing olefinswithout excessive olefin saturation which process comprises: a) reactingan olefin containing naphtha feedstream in a hydrodesulfurization zonecontaining a hydrodesulfurization catalyst comprising a catalyticdesulfurization metal selected from the group consisting of cobalt,nickel, molybdenum and tungsten on a refractory inorganic oxide supportwherein the desulfurization metal is surface impregnated such that theaverage concentration of the surface impregnated desulfurization metalon the surface layer having a thickness from about 40 to about 400microns is at least about two times the concentration of the respectivedesulfurization metal in the center core of the support; and b)recovering a naphtha stream having a reduced sulfur concentration. 14.The process of claim 13 wherein the refractory inorganic oxide supportcomprises gamma alumina, delta alumina, theta alumina, silica-alumina,zeolites, non-zeolitic molecular sieves, titania and mixtures thereof.15. The process of claim 13 wherein the hydrodesulfurization zone isoperated at conditions including a temperature from about 204° C. (400°F.) to about 399° C. (750° F.) and a pressure from about 790 kPa (100psig) to about 3950 kPa (500 psig).
 16. The process of claim 13 whereinthe naphtha feedstream boils in the range from about 38° C. (100° F.) toabout 232° C. (450° F.).